Thermally actuated switch

ABSTRACT

A thermally actuated, temperature compensated switch includes a bimetallic member and leaf spring members mounted on one or more printed circuit boards. The bimetallic member and the leaf spring members include lateral extensions extending through and secured to a printed circuit board. Means are provided for monitoring plural conditions in an electrical circuit and for changing the state of the thermally actuated switch upon the occurrence of a fault condition in the electrical circuit.

United States Patent 1 Dalziel et al.

[ Dec. 25, 1973 THERMALLY ACTUATED SWITCH [75] Inventors: Donald F. Dalziel, Sturtevant;

Charles H. Heide, Kenosha, both of Wis.

[73] Assignee: Webster Electric Company, Inc.,

Racine, Wis.

[22] Filed: Dec. 3, 1971 21 Appl. No.: 204,491

[52] US. Cl. 337/77, 317/101 CW, 337/100 [51] Int. Cl. l-l0lh 71/16 [58] Field of Search 337/70, 72, 75, 77,

[56] References Cited UNITEDSTATES PATENTS 3/1924 Zimmerman.....- ..337/77 3/1966 Randolph etal.-. ..337/81 3,496,509 2/1970 Metcalf 337/77 2,775,667 12/1956 Greenawalt... 337/77 3,304,468 2/1967 Lawson 317/101 CW 3,302,066 1/1967 Reimann 317/101 CW Primary ExaminerHarold Broome Attorney-Richard D. Mason et al.

[5 7 ABSTRACT A thermally actuated, temperature compensated switch includes a bimetallic member and leaf spring members mounted on one or more printed circuit boards. The bimetallic member and the leaf spring members include lateral extensions extending through and secured to a printed circuit board. Means are provided for monitoring plural conditions in an electrical circuit and for changing the state of the thermally actuated switch upon the occurrence of a fault condition in the electrical circuit.

24 Claims, 14 Drawing Figures 'PAIENTED m 25 ms 3. 781. 742 SHEET 1 [IF 2 INVENTO/"PS: DONALD E DALz/EL,

CHARLES H. H E/DE ATTORNEYS.

PMENTEBDECZS ms 3.781. 742

FIG. 14 i 14 LL) 7. lNl/E/VTORS: 22 K34 DONALD E DALZ/EL f CHARLES H. H5 05,

A7705 EYS.

THERMALLY ACTUATED SWITCH BACKGROUND OF THE INVENTION A. Field of the Invention The present invention relates generally to thermal delay and electrical overload actuated devices and more particularly to a switching mechanism used in such devices.

B. Description of the Prior Art Thermal delay switch mechanisms are often used in various types of ignition circuits, such as, ignition circuits for oil or gas burners. If ignition does not take place in a predetermined ignition trial period after an operating potential is applied to the ignition circuit, the thermal delay safety switch contacts are operated to deenergize the ignition circuit and to prevent the operation of the oil or the gas burner. Electrical overload actuated switch devices are used to monitor the operation of electrical circuits. For example, when an excessive amount of current is sensed by the electrical overload switch at a particular location in a circuit, switch contacts may be operated to deenergize the electrical circuit. Bimetallic elements are widely used in both thermal delay and electrical overload switch mechanisms. These'elements when placed in a heat sensing relation with a heating device'such as a resistor deform in response to heat resulting from a current flow through the resistor and operate a set of switch contacts to deenergize an electrical circuit or perform other functions. If a switch utilizing a bimetallic element is suitably temperature compensated,changes in ambient temperature do not affect the operation of the switch.

There is an ever present need for thermal delay or electrical overload actuated switches which are reliable, inexpensive, lightweight and simple in construction. Prior art switches of this type have been subject to disadvantages including elaborate and costly construction and unreliable operation. An example of a typical prior art temperature compensated switch mechanism is disclosed in US. Pat. No. 3,015,007 to J. F. Howard, dated Dec. 26, 1961.

SUMMARY OF THE INVENTION Objects 'of the present invention are the provision of a new and improved thermally actuated switch mechanism; the provision of a switch structure that can easily and compactly be associated with other components; the provision of an inexpensive, reliable, lightweight, thermally actuated, temperature compensated switch mechanism that is very simple in construction; the provision of a simple, I thermally actuated, temperature compensated switch assembly mounted on one or more printed circuit boards; the provision of novel means for mounting the various elements of a thermally actuated, temperature compensated switch assembly securely on one or more printed circuit boards; and the provision of a new and improved thermally actuated, temperature compensated switch assembly capable of monitoring plural operating conditions of an electrical circuit and capable of changing the state of the switch assembly in response to a fault occurring at either one or both of the operating conditions being monitored.

Briefly, the above and other objects of the present invention are achieved by a thermally actuated switch assembly mounted on one or more printed circuit boards. The switch assembly, in a preferred embodiment, in-

cludes a U-shaped bimetallic device having a freely movable end and a fixed end with lateral extensions passing through and soldered to at least one printed circuit board. The movable end of the bimetallic device holds a latching means for securely retaining one oftwo leaf spring members in place during the normal operation of the switch assembly. The switch assembly also includes separate resistors for detecting fault conditions arising at plural parts of an electrical circuit. The U-shaped bimetallic device is thermally coupled to each of the resistors and is deformable in response to a temperature increase caused by an excessive amount of current flow or by a flow of electrical current occurring for more than a predetermined period of time through one or both of the resistors to deenergize the electrical circuit being monitored. The thermally actuated, temperature compensated switch assembly advantageously permits the mounting of additional circuit elements on the same printed circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and advantages and novel features of the invention will appear from the fol lowing detailed description of a preferred embodiment of the invention illustrated in the accompanying drawings wherein:

FIG. 1 is a front perpsective view ofa thermally actuated, temperature compensated switch assembly embodying features of the present invention;

FIG. 2 is a rear perspective view of the switch assembly illustrated in FIG. 1;

FIG. 3 is an enlarged fragmentary top view of a portion of the switch mechanism taken from the line 3-3 of FIG. 2;

FIG. 4 is an elevational view of a portion of the switch mechanism taken from the line 4-4 of FIG. 3;

FIG. 5 is a sectional view of a portion of the switch mechanism taken along line 5-5 of FIG. 4;

FIG. 6 is a top plan view illustrating the switch assembly of FIGS. 1 and 2 in its normal operating condition;

FIG. 7 is a bottom plan view showing the side of the switch assembly opposite to the side illustrated in FIG.

FIG. 8 is a view similar to FIG. 6 illustrating the switch assembly after the detection of a fault condition;

FIG. 9 is a view similar to FIG. 6 illustrating the switch assembly in a high ambient temperature condition;

FIG. 10 is a view similar to FIG. 6 illustrating the switch assembly in a low ambient temperature condition;

FIG. 11 is a view similar to FIG. 6 illustrating the switch assembly during a reset operation;

FIG. 12 is a perspective view of a thermally actuated, temperature compensated switch assembly comprising an alternative embodiment of the present invention;

FIG. 13 is a side elevational view of the switch assembly of FIG. 12 taken from line l313 of FIG. 12;

FIG. 14 is a fragmentary sectional view of a portion of the switch assembly illustrated in FIG. 12 taken along line l4l4 of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings and initially to FIGS. 1, 2 and 6, there is illustrated a new and'improved thermally actuated switch assembly generally designated as 20 constructed in accordance with the principles of the present invention. The switch assembly serves to automatically monitor one or more operating parameters of an external electrical circuit and to deenergize the electrical circuit or perform another switching function in response to the detection of a fault condition.

In accordance with an important advantage of the present invention, the switch assembly 20 is mounted in novel fashion directly on a printed circuit board 22. Thus, the switch assembly 20 includes a switch actuator 24 that is used to open and close a pair of electrical switch contacts 26 and 28 secured to upper and lower leaf springs 30 and 32, respectively. The switch actuator 24, the upper leaf spring 30 and the lower leaf spring 32 are securely mounted on the printed circuit board 22 by mounting means generally designated as 34 and illustrated in detail in FIGS. 3, 4 and 5. The mounting means 34 includes lateral extensions 36, which in the illustrated embodiment are circular but which may take any other expedient shape, for example, a V-shape. The extensions 36 insert into and are soldered or otherwise attached to holes 38 in the printed circuit board 22.

In accordance with this aspect of the invention, the lateral extensions 36 act as springs biased outwardly against the edges or walls of the holes 38 in the printed circuit board 22 and provide mechanical support for the switch actuator 24, the upper leaf spring and the lower leaf spring 32 due to the springiness of the extensions 36. This mechanical support also can be used to hold the elements temporarily in place during assembly prior to soldering.

Illustrative of the additional components that may be mounted on the printed circuit board 22 in addition to the switch assembly 20 is a diode 40 (FIG. 2). By mounting the switch assembly 20 and, if desired, other components on the printedcircuit board 22, interconnections between the various elements of the switch assembly 20 and the other components mounted on the printed circuit board 22 and connections to an external electrical circuit may be conveniently made by means of the conductive strips 42 on the reverse side of the printed circuit board 22 (FIG. 7).

In an alternative embodiment of the present invention (FIGS. l2, l3 and 14), the switch assembly 20 may be mounted on two parallel printed circuit boards 22 and 44. The use of two parallel printed circuit boards 22 and 44 provides more ridigity of the entire assembly as well as additional space for mounting other circuit elements, and provides even greater flexibliity in making circuit connections among the various elements of the switch assembly 20 and between the elements of the switch assembly 20 and an external electrical circuit.

Proceeding to a more detailed description of the structure of the switch assembly 20, the assembly 20- H (FIG. 6) includes a switch actuator 24 comprising a U- formable in response to temperature changes. As is characteristic of bimetallic members, each of the bimetallic strips 50 and 52 have sides 54 and 56, respectively, exhibiting a higher coefficient of thermal expansion than their opposite sides to provide the desired deformation.

The operating condition of an external electrical circuit is monitored by the switch assembly 20 by sensing the electrical current flow through one or both of the plural resistors 58 and 60 mounted in proximity to the bimetallic strip 52 of the switch actuator 24. The resistor 58 may advantageously be of a flexible type and may be mounted directly on the bimetallic strip 52. Since the U-shaped bimetallic member 46 is, in a preferred embodiment, highly electrically conductive, one end of the resistor 58 is electrically connected to the U-shaped bimetallic member 46 which thus serves as either a lead to or a lead from the resistor 58. The resistor 60 may advantageously be ofa fixed type and may be mounted to the printed circuit board 22 and thermally connected in a novel manner to the bimetallic strip 52 by a flexible, thermally conductive heat transfer strip 62.

FIG. 6 illustrates the switch assembly 20 in its normal operating condition with the freely movable end of the tion.

More specifically, by constructing the leaf spring member 30 and 32 of a highly conductive metal, a series connected electrical circuit switch is formed by applying an electrical current to the fixed end of the upper leaf spring 30 for passage through the upper leaf spring 30, the electrical contact 26, the electrical contact 28 andthe lower leaf spring 32. Thus, when the freely movable end of the upper leaf spring 30 is released by the latch 48 inresponse to heat due to a fault condition, the electrical contacts 26 and 28 separate to open the electrical circuit switch and to prevent the flow of electrical current therethrough.

The bimetallic strip 52 of the switch actuator 24 is heated by an electrical current flowing through either one or both of the resisstors 58 and 60. In a preferred embodiment, the bimetallic strip 52 is gradually heated and expanded in response to a current flowing through the resistive element 58. The heating and expansion of the bimetallic strip 52 continues as long as an electrical current flows through the resistive element 58. After I the predetermined thermal time delay period of the bimetallic strip 52 has expired, the bimetallic strip 52 is deformed to such a degree that the latch 48 is moved out of engagement with the freely movable end of the upper leaf spring 30 causing the electrical contacts 26 and 28 to open. This manner of operation enables the switch assembly 20 to be used as a thermal delay switch.

The switch assembly 20 may also be used as an electrical overload actuated switch. The bimetallic strip 52 may be heated and expanded by the resistive element 60 through the heat transfer strip 62. An electrical current flowing through the resistive element 60 generates heat which is transmitted to the conductive strip 62. Under normal operating conditions, the heat generated and transmitted to the conductive strip 62 will dissipate before an appreciable amount of the heat is transferred to the bimetallic strip 52. During a fault condition (FIG. 8), as when an electrical short occurs in a portion of an electrical circuit being monitored, an unduly large electric current flows through the resistor 60 thereby generating a significantly greater amount of heat. Due to the magnitude of the heat generated, a large portion thereof is transferred by the strip 62 to the bimetallic strip 52 causing the bimetallic strip 52 to heat and to expand. The bimetallic strip 52 continues to heat and expand until the latch 48 moves out of engagement with the freely movable end of the upper leaf spring 30 causing the electrical contacts 26 and 28 to open (FIG. 8) and thereby deenergize the electrical circuit being monitored.

Preferably the switch assembly 20 is compensated for changes in its ambient temperature by forming the U- shaped bimetallic member 46 of the switch actuator 24 with the two L-shaped bimetallic strips 50 and 52. Since the sides 54 and 56 ofthe bimetallic strips 50 and 52 have higher coefficients of thermal expansion than the opposite sides, as the side 56 of the bimetallic strip 52 expands in response to an increase in the ambient temperatures (FIG. 9), the side 54 of the bimetallic strip 50 also expands to maintain the latch 48 in substantially the same position that it occupies under nor.- mal ambient temperature conditions (FIG. 6). In response to a low ambient temperature condition (FIG. 10), side 54 of the bimetallic strip 50 contracts sufficiently to counteract the contraction of side 56 of the bimetallic strip 52 thereby'maintaining the latch 48 in substantially the same position that it occupies under normal ambient temperature conditions. Thus, in all ambient conditions the latch 48 securely maintains the 'freely movable end of the upper leaf spring 30 in engagement to prevent the opening of the electrical contacts 26 and 28 in the absence of a fault condition. Further, the ambient temperature compensation of the switch assembly more importantly maintains the thermal time delay period ofthe bimetallic strip 52 substantially constant, regardless of ambient temperature variations.

The switch assembly 20 is provided with a reset structure generally designated as 64 (FIG. 11) for resetting the switch assembly 20 to its normal operating condition (FIG. 6) after the detection of a' fault condition (FIG. 8). The reset means 64 includes a pushbutton reset plunger 66 (FIG. 11) adapted to be pressed by an operator to return the switch assembly 20 to its normal operating condition. In response to a fault condition, the freely movable end of the upper leaf spring 30 is released from engagement with the latch 48 and springs into engagement with an enlarged diameter portion 68 of the pushbutton reset plunger 66. The lower leaf spring 32 is held in place by a reduced diameter portion 70 of the pushbutton reset plunger 66 to thereby maintain the electrical contacts 26 and 28 in an open condition. The electrical contacts 26 and 28 are maintained in an open condition during the reset operation (FIG. 11) by the enlarged and reduced diameter portions 68 and 70 of the pushbutton reset plunger 66 and by an aperture 72 (FIGS. 2 and 11) in the upper leaf spring 30 that permits the reduced diameter portion of the pushbutton reset plunger 66 to pass therethrough. Assuming that the bimetallic strip 52 has sufficiently cooled to return the latch 48 to its normal operating position (FIG. 11), the switch assembly 20 is reset by manually depressing the pushbutton reset plunger 66 to return the freely movable end of the upper leaf spring 30 into secure engagement with the latch 48. Upon release of the pushbutton reset plunger 66, the electrical contacts 26 and 28 return to a closed condition. If the bimetallic strip 52 has not sufficiently cooled to return the latch 48 to its normal operating condition, depressing the pushbutton reset plunger 66 does not result in the closing of the electrical contacts 26 and 28. Thus, the switch assembly 20 cannot be reset until the bimetallic strip 52 has sufficiently cooled to return the latch 48 to its normal operating condition (FIG. 11).

Many modifications and variations of the present invention are possible. For example, the switch assembly 20 (FIG. 6) may be modified so that the electrical contacts 26 and 28 are normally open in the normal operating condition of the switch assembly 20. This may simply be achieved by shortening the length of the upper leaf spring 30 to prevent its engagement by the latch 48 and by increasing the length of the lower leaf spring 32 to permit its freely movable end to be securely engaged by the latch 48 in the normal operating condition of the switch assembly 20. The reduced diameter end portion of the pushbutton reset plunger 66 and the need for an aperture 72 in the upper leaf spring 30 is eliminated in the normally open embodiment of the switch assembly 20. Upon the detection of a fault condition in an external electrical circuit being monitored, the freely movable end of the lower leaf spring 32 is released from its engagement with the latch 48 enabling the electrical contacts 26 and 28 to come into contact and to complete an electrical circuit to, for example, sound an audible or visible alarm.

Additionally, a second flexible resistor might be mounted on the bimetallic strip 52, on the side opposite resistor 58 (FIG. 6). Alternatively, by suitably changing the dimensions of the U-shaped bimetallic member 46, a second fixed resistor 60 thermally connected to the bimetallic strip 52 by a second flexible thermally conductive strip 62 may be used in place of the flexible resistive element 58. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim:

I. A thermally actuated switch comprising in combination:

a printed circuit board,

first and second movable leaf spring members disposed generally parallel to one another and generally transverse to said board,

cooperating switch contact means on said first and second leaf spring members,

a movable bimetallic member disposed generally transverse to said board and including means engageable with at least one of said leaf spring members for controlling the operation of said switch contact means,

heating means adjacent said bimetallic member for initiating and controlling the movement of said bimetallic member from an unheated or at-rest position,

integral extensions of said bimetallic member and of said leaf spring members extending through said board, said integral extensions of said bimetallic member comprising means for opposing the movement of said bimetallic member away from said atrest postion and for providing a bias force to return said bimetallic member to said at-rest position,

and means forming a solder bond between said board and said extensions for mounting said bimetallic member and said leaf spring members in position on said board.

2. The thermally actuated switch recited in claim 1 in which said bimetallic member and said first and second leaf spring members are mounted on and between two printed circuit boards.

3. The thermally actuated switch recited in claim 1 in which the cooperating switch contact means are disposed to form a normally closed switch.

4. The thermally actuated switch recited in claim 1 in which the integralextensions comprise lateral extensions of said bimetallic member and of said leaf spring members.

5. The thermally actuated switch recited in claim 1 in which said heating means comprise at least one resistive element.

6. A thermally actuated switch comprising in combination: 1

a printed circuit board,

first and second leaf spring members disposed generally parallel to one another and generally transverse to said board,

cooperating switch contact means on said first and second leaf spring members,

a bimetallic member disposed generally transverse to said board. and including means engageable with at least one of said leaf spring members for controlling the operation of said switch contact means,

heating meansadjacent said bimetallic member for controlling the movement of said bimetallic member,

- integral extensions of said bimetallic member and of said leaf spring members extending through said board,

and means forming a solder bond between said-board and said extensions for mounting said bimetallic member and said leaf spring members in position on said board,

said heating means comprising plural resistive elements, one of said plural resistive elements being mounted on said bimetallic member and another of said plural resistive elements being mounted on said printed circuit board and thermally connected to said bimetallic member by a thermally conductive strip, a first portion of said conductive strip being in contact with said fixed resistive element and a second portion of said conductive strip being in contact with said bimetallic member.

7. The thermally actuated switch recited in claim 1 wherein said bimetallic member comprises two cooperating bimetallic strips securely fastened together at one a portion of said bimetallic member being movable away from an at'rest position;

means for mounting said bimetallic member on said printed circuit board, said mounting means com prising integral extensions of said bimetallic member disposed in holes in said printed circuit board, said extensions providing a bias force in opposition to the movement of said movable portion of said bimetallic member away from said at-rest position;

plural contact means for enabling the openingand closing of an electrical circuit mounted on said printed circuit board;

latching means responsive to said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes;

and means for heating said bimetallic member to cause the movement of said movable portion of said bimetallic member away from said at-rest position and to cause said latching means to release said one of said plural contact means thereby enabling the switch to enter the second of two switching modes after a predetermined amount of movement of said movable portion of said bimetallic member away from said at-rest position.

9. A thermally actuated switch comprising:

a printed circuit board;

a bimetallic member having a predetermined thermal time delay mounted on said printed circuit board, a portion of said bimetallic member being freely movable;

plural contactmeans for enabling the opening and closing of an electrical circuit mounted on said printed circuit board;

latching means responsive to said bimetallic member for releasably-retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes; 7 I

and means for heating said bimetallic member to cause said latching means to release said one of said plural contact means thereby enabling the switch to enterthe second of two switching modes,

said bimetallic member comprising a generally U- shaped bimetallic assembly having two bimetallic strips fixedly secured together alon'gan end portion of each strip, the other end portion of the first of said strips being securely mounted on said printed circuit board and the other end portion of the second of two bimetallic strips being freely movable with respect to said other end portion of said first of said two bimetallic strips, wherein said two bi.- metallic strips compensate for changes in the ambient temperature to which said thermally actuated switch is exposed such that said changes are ineffective to cause said thermally actuated switch to change from one switching mode to another switching mode and such that said predetermined thermal time delay period remains substantially constant throughout said changes.

10. The thermally actuated switch recited in claim 9 switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second mode while resetting said thermally actuated switch to said first mode.

12. The thermally actuated switch recited in claim 8 wherein said heating means comprise first and second heating devices located in proximity to said bimetallic member, each of said heating devices being individually capable of causing said thermally actuated switch to change from said first switching mode to said second switching mode.

13. A thermally actuated switch comprising:

a printed circuit board;

a bimetallic member having a predetermined thermal time delay mounted on said printed circuit board, a portion of said bimetallic member being freely movable;

plural contact means for enabling the opening and closing of an electrical circuit mounted on said printed circuit board;

latching means responsive to said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes;

and means for heating said bimetallic member to cause said latching means to release said one of said plural contact means thereby enabling the switch to enter the second of two switching modes,

said heating means comprising first and second heating devices located in proximity to said bimetallic member, each of said heating devices being individually capable of causing said thermally actuated switch to change from said first switching mode to said second switching mode, wherein at least one of said two heating devices is thermally connected to said bimetallic member by means of a thermally conductive metallic strip, one portion of said thermally conductive metallic strip being in contact with one of said two heating devices and a second portion of said thermally conductive metallic strip being in contact with said bimetallic member.

14. A thermally actuated switch comprising:

a printed circuit board;

a bimetallic member having a predetermined thermal time delay period mounted on said printed circuit board having a freely movable end portion;

plural movable contact means mounted on said printed circuit board for enabling the opening and closing of an electrical circuit;

latching means attached to the movable end portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the thermally actuated switch in the first of two switching modes;

first and second means for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing the latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes;

and means associated with said bimetallic member to compensate for changes in the ambient temperature to which said thermally actuated switch is exposed to prevent said changes from causing said thermally actuated switch to change from one switching mode to another switching mode, said compensating means further being effective to maintain said thermal time delay period substantially constant throughout said ambient temperature changes; and

means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of two switching modes while resetting said thermally actuated switch to said first of said two switching modes.

l5. A thermally actuated switch comprising:

a printed circuit board;

a bimetallic member having a predetermined thermal time delay period mounted on said printed circuit board having a freely movable end portion;

plural movable contact means mounted on said printed circuit board for enabling the opening and closing of an electrical circuit;

latching means attached to the movable end portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the thermally actuated switch in the first of two switching modes;

first and second means for heating said bimetallic member located in proximity to said bimetallic member each of said first and second means being individually capable of causing the latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes;

and means associated with said bimetallic member to compensate for changes in the ambient temperature to which said thermally actuated switch is exposed to prevent said changes from causing said thermally actuated switch to change from one switching mode to another switching mode, said compensating means further being effective to maintain said thermal time delay period substantially constant throughout said ambient temperature changes,

one of said two heating means being thermally connected to said bimetallic member by means of a thermally conductive metallic strip extending from said one of said two heating means to said bimetallic member.

16. A thermally actuated switch comprising:

plural contact means for enabling the opening and the closing of an electrical circuit,

a bimetallic member having a predetermined thermal time delay and including a movable portion,

latching means responsive to said movable portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes,

first and second means for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing said latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes and means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of two switching modes while resetting said thermally actuated switch to said first of said two switching modes.

17. A thermally actuated switch comprising:

plural contact means for enabling the opening and the closing of an electrical circuit, a bimetallic member having a predetermined thermal time delay and including a movable portion,

latching means responsive to said' movable portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes and first and secondmeans for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing said latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes,

said first heating means including means for enabling said latching means to release one of said plural contact means to thereby place said switch in the second of said two switching modes in response to the duration of a signal in excess of a predetermined time period,

said second heating means including means for enabling said latching means to release one of said plural contact means to thereby place said switch in the second of said two switching modes in response to the presence of a signal having a magnitude in excess of a predetermined level and for simultaneously inhibiting said latching means from releasing said one of said plural contact means in response to the presence of a signal having a magnitude less than said predetermined level.

18. The thermally actuated switch recited in claim 17 wherein said first heating means comprises a resistive elementmounted on said bimetallic member.

19. The thermally actuated switch recited in claim 17 wherein said second heating means comprises a resistive element positioned adjacent said bimetallic memher and thermally connected to said bimetallic member by a thermally conductive strip, a'first portion of said conductive strip being in contact with said resistive element and a second portion of said conductive strip being in contact with said bimetallic member.

20. A thermally actuated switch comprising plural contact means for enabling the opening and the closing of an electrical circuit, a bimetallic member having a predetermined thermal time delay and including a movable portion,

latching meansresponsive to said movable'portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes, and

means for heating said bimetallic member to cause said latching means to release one of said plural contact means thereby enabling said switch to enter the second of said two switching modes, said heating means comprising a resistive element positioned adjacent said bimetallic member and thermally connected to said bimetallic member by a thermally conductive strip, a first portion of said conductive strip being in contact with said resistive element and a second portion of said conductive strip being in contact with said bimetallic member.

21. The thermally actuated switch recited in claim 20 further comprising means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of said two switching modes while resetting said thermally actuated switch to said first of said two switching modes.

22. The thermally actuated switch recited in claim 20 further comprising a first printed circuit board, said bimetallic member being mounted on said first printed circuit board.

23. The thermally actuated switch recited in claim 22 wherein said bimetallic member includes integral extensions extending through said first printed circuit board and further comprising means forming a solder bond between said first printed circuit board and said extensions for mounting said bimetallic member in position on said first printed, circuit board.

24. The thermally actuated switch recited in claim 22 further comprising a second printed circuit board, said bimetallic member being mounted on and between said first and said second printed circuit boards. 

1. A thermally actuated switch comprising in combination: a printed circuit board, first and second movable leaf spring members disposed generally parallel to one another and generally transverse to said board, cooperating switch contact means on said first and second leaf spring members, a movable bimetallic member disposed generally transverse to said board and including means engageable with at least one of said leaf spring members for controlling the operation of said switch contact means, heating means adjacent said bimetallic member for initiating and controlling the movement of said bimetallic member from an unheated or at-rest position, integral extensions of said bimetallic member and of said leaf spring members extending through said board, said integral extensions of said bimetallic member comprising means for opposing the movement of said bimetallic member away from said at-rest postion and for providing a bias force to return said bimetallic member to said at-rest position, and means forming a solder bond between said board and said extensions for mounting said bimetallic member and said leaf spring members in position on said board.
 2. The thermally actuated switch recited in claim 1 in which said bimetallic member and said first and second leaf spring members are mounted on and between two printed circuit boards.
 3. The thermally actuated switch recited in claim 1 in which the cooperating switch contact means are disposed to form a normally closed switch.
 4. The thermally actuated switch recited in claim 1 in which the integral extensions comprise lateral extensions of said bimetallic member and of said leaf spring members.
 5. The thermally actuated switch recited in claim 1 in which said heating means comprise at least one resistive element.
 6. A thermally actuated switch comprising in combination: a printed circuit board, first and second leaf spring members disposed generally parallel to one another and generally transverse to said board, cooperating switch contact means on said first and second leaf spring members, a bimetallic member disposed generally transverse to said board and including means engageable with at least one of said leaf spring members for controlling the operation of said switch contact means, heating means adjacent said bimetallic member for controlling the movement of said bimetallic member, integral extensions of said bimetallic member and of said leaf spring members extending through said board, and means forming a solder bond between said board and said extensions for mounting said bimetallic member and said leaf spring members in position on said board, said heating means comprising plural resistive elements, one of said plural resistive elements being mounted on said bimetallic member and another of said plural resistive elements being mounted on said printed circuit board and thermally connected to said bimetallic member by a thermally conductive strip, a first portion of said conductive strip being in contact with said fixed resistive element and a second portion of said conductive strip being in contact with said bimetallic member.
 7. The thermally actuated switch recited in claim 1 wherein said bimetallic member comprises two cooperating bimetallic strips secUrely fastened together at one end thereof to effect compensation for the changes in the ambient temperature to which said switch is exposed.
 8. A thermally actuated switch comprising: a printed circuit board; a bimetallic member having a predetermined thermal time delay mounted on said printed circuit board, a portion of said bimetallic member being movable away from an at-rest position; means for mounting said bimetallic member on said printed circuit board, said mounting means comprising integral extensions of said bimetallic member disposed in holes in said printed circuit board, said extensions providing a bias force in opposition to the movement of said movable portion of said bimetallic member away from said at-rest position; plural contact means for enabling the opening and closing of an electrical circuit mounted on said printed circuit board; latching means responsive to said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes; and means for heating said bimetallic member to cause the movement of said movable portion of said bimetallic member away from said at-rest position and to cause said latching means to release said one of said plural contact means thereby enabling the switch to enter the second of two switching modes after a predetermined amount of movement of said movable portion of said bimetallic member away from said at-rest position.
 9. A thermally actuated switch comprising: a printed circuit board; a bimetallic member having a predetermined thermal time delay mounted on said printed circuit board, a portion of said bimetallic member being freely movable; plural contact means for enabling the opening and closing of an electrical circuit mounted on said printed circuit board; latching means responsive to said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes; and means for heating said bimetallic member to cause said latching means to release said one of said plural contact means thereby enabling the switch to enter the second of two switching modes, said bimetallic member comprising a generally U-shaped bimetallic assembly having two bimetallic strips fixedly secured together along an end portion of each strip, the other end portion of the first of said strips being securely mounted on said printed circuit board and the other end portion of the second of two bimetallic strips being freely movable with respect to said other end portion of said first of said two bimetallic strips, wherein said two bimetallic strips compensate for changes in the ambient temperature to which said thermally actuated switch is exposed such that said changes are ineffective to cause said thermally actuated switch to change from one switching mode to another switching mode and such that said predetermined thermal time delay period remains substantially constant throughout said changes.
 10. The thermally actuated switch recited in claim 9 wherein said other end portion of said first of said two bimetallic strips is securely anchored in said printed circuit board by means of at least one lateral extension of said first of said two bimetallic strips.
 11. The thermally actuated switch recited in claim 8 further comprising means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second mode while resetting said thermally actuated switch to said first mode.
 12. The thermally actuated switch recited in claim 8 wherein said heating means comprise first and second heating devices located in proximity to said bimetallic member, each of said heating devices being individually capable of causing said thermally actuated switch to change from said first switching mode to said second switching mode.
 13. A thermally actuaTed switch comprising: a printed circuit board; a bimetallic member having a predetermined thermal time delay mounted on said printed circuit board, a portion of said bimetallic member being freely movable; plural contact means for enabling the opening and closing of an electrical circuit mounted on said printed circuit board; latching means responsive to said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the switch in the first of two switching modes; and means for heating said bimetallic member to cause said latching means to release said one of said plural contact means thereby enabling the switch to enter the second of two switching modes, said heating means comprising first and second heating devices located in proximity to said bimetallic member, each of said heating devices being individually capable of causing said thermally actuated switch to change from said first switching mode to said second switching mode, wherein at least one of said two heating devices is thermally connected to said bimetallic member by means of a thermally conductive metallic strip, one portion of said thermally conductive metallic strip being in contact with one of said two heating devices and a second portion of said thermally conductive metallic strip being in contact with said bimetallic member.
 14. A thermally actuated switch comprising: a printed circuit board; a bimetallic member having a predetermined thermal time delay period mounted on said printed circuit board having a freely movable end portion; plural movable contact means mounted on said printed circuit board for enabling the opening and closing of an electrical circuit; latching means attached to the movable end portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the thermally actuated switch in the first of two switching modes; first and second means for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing the latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes; and means associated with said bimetallic member to compensate for changes in the ambient temperature to which said thermally actuated switch is exposed to prevent said changes from causing said thermally actuated switch to change from one switching mode to another switching mode, said compensating means further being effective to maintain said thermal time delay period substantially constant throughout said ambient temperature changes; and means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of two switching modes while resetting said thermally actuated switch to said first of said two switching modes.
 15. A thermally actuated switch comprising: a printed circuit board; a bimetallic member having a predetermined thermal time delay period mounted on said printed circuit board having a freely movable end portion; plural movable contact means mounted on said printed circuit board for enabling the opening and closing of an electrical circuit; latching means attached to the movable end portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position placing the thermally actuated switch in the first of two switching modes; first and second means for heating said bimetallic member located in proximity to said bimetallic member each of said first and second means being individually capable of causing the latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes; and means associated with said biMetallic member to compensate for changes in the ambient temperature to which said thermally actuated switch is exposed to prevent said changes from causing said thermally actuated switch to change from one switching mode to another switching mode, said compensating means further being effective to maintain said thermal time delay period substantially constant throughout said ambient temperature changes, one of said two heating means being thermally connected to said bimetallic member by means of a thermally conductive metallic strip extending from said one of said two heating means to said bimetallic member.
 16. A thermally actuated switch comprising: plural contact means for enabling the opening and the closing of an electrical circuit, a bimetallic member having a predetermined thermal time delay and including a movable portion, latching means responsive to said movable portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes, first and second means for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing said latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes and means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of two switching modes while resetting said thermally actuated switch to said first of said two switching modes.
 17. A thermally actuated switch comprising: plural contact means for enabling the opening and the closing of an electrical circuit, a bimetallic member having a predetermined thermal time delay and including a movable portion, latching means responsive to said movable portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes and first and second means for heating said bimetallic member located in proximity to said bimetallic member, each of said first and second means being individually capable of causing said latching means to release one of said plural contact means, thereby enabling the thermally actuated switch to enter the second of said two switching modes, said first heating means including means for enabling said latching means to release one of said plural contact means to thereby place said switch in the second of said two switching modes in response to the duration of a signal in excess of a predetermined time period, said second heating means including means for enabling said latching means to release one of said plural contact means to thereby place said switch in the second of said two switching modes in response to the presence of a signal having a magnitude in excess of a predetermined level and for simultaneously inhibiting said latching means from releasing said one of said plural contact means in response to the presence of a signal having a magnitude less than said predetermined level.
 18. The thermally actuated switch recited in claim 17 wherein said first heating means comprises a resistive element mounted on said bimetallic member.
 19. The thermally actuated switch recited in claim 17 wherein said second heating means comprises a resistive element positioned adjacent said bimetallic member and thermally connected to said bimetallic member by a thermally conductive strip, a first portion of said conductive strip being in contact with said resistive element and a second portion of said conductive strip being in contact with said bimetallic member.
 20. A thermally actuated switch comprising plural contact means for enabling the opening and the closing of an electrical circuit, a bimetallic member having a predetermIned thermal time delay and including a movable portion, latching means responsive to said movable portion of said bimetallic member for releasably retaining one of said plural contact means in a generally fixed position to place said switch in the first of two switching modes, and means for heating said bimetallic member to cause said latching means to release one of said plural contact means thereby enabling said switch to enter the second of said two switching modes, said heating means comprising a resistive element positioned adjacent said bimetallic member and thermally connected to said bimetallic member by a thermally conductive strip, a first portion of said conductive strip being in contact with said resistive element and a second portion of said conductive strip being in contact with said bimetallic member.
 21. The thermally actuated switch recited in claim 20 further comprising means for manually resetting said switch from said second mode to said first mode and for maintaining said thermally actuated switch in said second of said two switching modes while resetting said thermally actuated switch to said first of said two switching modes.
 22. The thermally actuated switch recited in claim 20 further comprising a first printed circuit board, said bimetallic member being mounted on said first printed circuit board.
 23. The thermally actuated switch recited in claim 22 wherein said bimetallic member includes integral extensions extending through said first printed circuit board and further comprising means forming a solder bond between said first printed circuit board and said extensions for mounting said bimetallic member in position on said first printed circuit board.
 24. The thermally actuated switch recited in claim 22 further comprising a second printed circuit board, said bimetallic member being mounted on and between said first and said second printed circuit boards. 